Laser, as a light source with high brightness and strong directivity, can emit a monochromatic coherent beam. And its various merits have given rise to an ever increasing utilization of laser as a light source in the field of projection display technologies in recent years. At present, however, the laser source is confronted with at least the following challenges in application:
One is high coherence of laser, which inevitably leads to a speckle effect. The so-called speckle refers to the phenomenon that, when a coherent light source irradiates on objects with rough surfaces, the light is scattered; with the same wavelength and a constant phase, the scattered light will generate interference in space, with constructive interference occurring at some positions and destructive interference occurring at some other positions in space, creating an overall effect of dark and bright granular speckles on a screen. Long-time watching at these unfocused flashing speckles will subject an observer to a sense of vertigo, and the projected image will also suffer a decline in quality, thus degrading the user's viewing experience, therefore a speckle-removing optical path is required to be designed for performing decoherence of laser beams to improve projection display effect of laser.
The second problem is that, in a laser propagation optical path, the cross-section area of an optical lens is typically much smaller than the area of laser speckles emitted by a laser device group (the diameter of the speckles generated by a laser device can be as large as roughly 60 mm, but the diameter of an ordinary optical lens is relatively small, for example, may be 30 mm or so, and the functioning area is even smaller, given that only a central part of an optical lens serves as a main region for processing light, due to factors such as processing, installation processes), therefore, laser beams generated by the laser device need to undergo a beam-shrinking process to form smaller speckles, only by which the transmittance rate and light processing efficiency of the beams can be enhanced when the beams pass through the optical lens. This is particularly true in the case of a hybrid laser source which is composed of laser and fluorescence, where fluorescent powder on a fluorescent wheel demands high-energy laser speckles to be excited, thus the diameter of laser speckles irradiating on the surface of the fluorescent wheel is about 0.8 mm, smaller than one-hundredth of the area of the laser speckles generated by the laser device, hence subjecting the laser speckles to the beam-shrinking process during laser propagation is highly necessary.
According to a laser speckle-removing optical component, laser beams emitted by a laser device firstly pass through a beam-shrinking component, which can be a lens or a lens group with light converging effect, then pass through a speckle-removing component arranged behind the beam-shrinking component, such as a diffusion sheet or a diffusion plate, for being successively processed by beam-shrinking and speckle-removing of laser, and then the laser beams are incident to a subsequent optical lens.
However, the technical personnel finds in application that, in the above laser speckle-removing optical component, transmittance of laser is reduced due to a large amount of dust particles collected by the diffusion sheet component, which not only deteriorates the speckle-removing effect, but also leads to a light attenuation phenomenon during laser propagation, great reduction in the brightness of the laser projection display image, shortened service life of a laser illumination light source. Therefore, what is said above poses a technical problem to be solved urgently in the application of laser source.